Generally speaking, such systems must provide an automatic locking when the movable element is brought into the position in which it must be locked, the unlocking being operated by a manual control means such as a pushing device.
Known locking systems comprise a hook fastened to one element as well as a movable interlocking member which is resiliently retractable and is mounted onto the other element in order to cooperate with the hook. For example the interlocking member can be a turning lever which comprises a bent back end part, bevelled so that the lever swings out when this end part enters into contact with the hook, and which then gets caught by the hook as a result of a pivoting in the opposite direction, while being retracted by a spring.
The unlocking is assured by acting on the lever, against the tension of the spring, with the help of a pushing device which causes the pivoting of the lever in order to uncouple its end from the hook. The pushing device can be made out of the same piece as the lever, or it can be constituted by a separate piece. In other systems, the interlocking member, instead of swinging out, is movable by translational motion, and it is also pushed by a spring to engage itself over the hook and provide the interlocking of the two elements.
Some of these systems require a huge number of parts: lever, swivelling axis, spring, pushing device, etc., and this is increasing their price and is rendering the assembling complicated. Others show only an unreliable behaviour in case of an impact, due to the fact that in such a case the extremity of the lever can be withdrawn from the hook.
One also knows, mainly from the document DE-A-3421564, a locking system in which the lock is constituted by an elastically deformable wire, which is connected by one end to a wall of an element of a seat and which extends itself against the said wall. A shiftable part of the wire stretches itself out across a window provided in the wall and cooperates with the hook when the latter is engaged into the window. The unlocking is operated by bending the wire so as to disengage it from the hook. It mainly results therefrom that the distance between the window and the clamping of the wire must be relatively important in order to allow a sufficient release of the wire as a result of this deflection, this entailing an important space requirement.
Moreover one knows, from the document WO-A-8102549, a locking system in which the lock is constituted by a wire worked so as to have the shape of a kind of ring mounted in a pivoting manner in a bearing connected to the base plate of a seat cushion, the swiwel axis being parallel to said base plate. A first part of this ring extends in a radial vertical plane and comprises a section which is parallel to the swivel axis and which is used as the element for the locking over a hook fastened to the floor of the vehicle. Another part of the ring extends also radially, perpendicular to the first part, and constitutes a lever, operated by an unlocking handle, in order to achieve the pivoting of the ring and to disengage the said first part from the hook. The two ends of the ring extend in parallel one with respect to the other in the direction of the swivel axis and are maintained in a fixed position on the base plate. The retracting of the ring from its locking position is provided by the rotating couple resulting from the deformation of the ring during its pivoting. All the strains tending to lift up the seat cushion are born by the ring itself, that transmits them to the bearing and to the clampings of the ends, and these strains result in important bending constraints in the wire of the ring, liable to entail its deformation and poor locking.